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7 Interpreting Measured I-V Curves
7-17
steeper. The shunt current in a series of modules or within a single module can be
dominated by a single hotspot on a single cell, or may arise from several smaller shunt
paths in several series cells.
Shunts within a module can improve over time, or can degrade until the module is
damaged irreparably. Smaller shunts can self-heal if the high current through the shunt
path causes the small amount of material shorting the cell to self-immolate. Larger shunts
can result in localized temperature rises in the module that can reach the melting point of
encapsulant material or the module backsheet. Modules that have failed in this manner
will tend to show burn spots or other obvious evidence of failure. Bypass diodes in the
PV module are designed to prevent damage due to hotspot, and so failure of the bypass
diode may accompany hotspot damage.
If the I-V measurement of a PV string shows a substantial slope, you can localize the
problem by successively breaking the string into smaller segments and measuring the
segments individually. Be sure to update the model with the reduced number of modules
in series.
Module I
sc
Mismatch
Increased slope along the upper leg of the I-V curve may have less to do with shunt
resistance, and more to do with small mismatches between the I
sc
values of each module.
I
sc
values in a real PV system will have some mismatch, due to slight manufacturing
variations, slightly different installation angles, or special cases of shading and non-
uniform soiling.
Special cases of shading can also cause more slope in the upper leg of the I-V curve. The
most common case takes place in multi-row tip-up arrays in which the upper edge of one
row of modules casts a sliver of shadow across the lower edge of another string of
modules. If the sliver of shade varies in height from one end of the shaded string to the
other, the result is an effective change in module I
sc
from one module to another. This can
cause the upper leg of the I-V curve to tilt more steeply.
Special cases of module soiling can also cause more slope in the upper leg of the I-V
curve. The most common case appears in shallow-tilt arrays that encourage soiling to
build up along the lower edge of modules. If a string of modules is mounted in a row and
the lower edge of the string is not horizontal, the height of the dirt band may vary from
one end of the string to the other. This has the same effect as a slight reduction in module
I
sc
from one end of the string to the other. The result is an increase in slope of the upper
leg of the I-V curve.
Tapered shade or dirt dam
When a string of modules is mounted in portrait orientation, a band of shade or dirt
across the entire string can cause the steeper slope in the horizontal leg of the curve, but
only if the band is tapered from one end to the other (or if there is a randomized slight
variation in the extent of the dirt band across the modules in the string).